The present invention relates in general to a method of producing a printed circuit board having through via conductors each having a through hole, and more specifically to a method of producing a printed circuit board wherein through holes of through via conductors are closed by resin fillings. The present invention further relates to a mask for carrying out the above described method.
Heretofore, for connection between wiring layers formed on the upper and lower surfaces of a printed circuit board, through via conductors have generally been used. In production of such a printed circuit board, the through holes of the through via conductors are closed by resin fillings so that an insulation resin layer formed over the through via holes can be flat or planar.
In this instance, the following method is known for closing the through holes of the through via conductors by resin fillings. In the first place, as shown in FIG. 10, a base 51 consisting of a substrate 50 and metal layers 52 covering the opposite upper and lower surfaces (though only the upper surface 51A is shown) of the substrate 50 is prepared. The base 51 has a plurality of through via conductors 55 extending completely between the upper surface 51A and lower surface of the base 51. Each through via conductor 55 has a through hole 54 defined by the inner circumferential surface thereof. A mask 61 having filling holes 62 aligned with the respective through holes 54 is placed on the surface 51A (printing surface). The through holes 54 are filled with resin paste by printing by the use of the mask 61.
After the filling by printing, the base 51 is heated to cure the resin paste fi lings 57 and thereby form cured resin fillings 58 in the through holes 54 as shown in FIG. 11.
Generally, the through holes 54 of the through via conductors 55 formed in the base 51 are not arranged at constant intervals but varying intervals. If the through holes 54 are filled with the same amount of resin paste 57, a larger amount of resin paste 57 per unit area is applied to a surface 51A region where the distance between adjacent two of the through holes 54 is smaller (the left-hand portion in FIG. 10) as compared with that applied to a surface 51A region where the distance between adjacent two of the through holes 54 is larger (the right-hand portion in FIG. 11).
In this instance, at the time of curing by heating, the resin paste 57 is once liquefied and spread over while wetting the surface 51A of the base 51, and thereafter cured. By observation of the surface 51A of the base 51 after the curing by heating, it will be found, as shown in FIG. 11, that the cured resin 58 in the surface 51A region where the distance between adjacent two of the through holes 54 is smaller protrudes from the surface 51A of the base 51 more than that in the surface 51A region where the distance between adjacent two of the through holes 54 is larger.
For this reason, when the unnecessary portion of the cured resin 58 is removed by grinding for thereby uncovering the surface of the metal layer 52, much time is required for removing the largely protuberant portion of the cured resin 58 (the left-hand portion in FIG. 11) or it may possibly occur such a case wherein the largely protuberant portion cannot be ground sufficiently, thus allowing some portion of the cured resin 58 to remain unremoved. Further, in case the surface 58A of the cured resin 58 has large undulations, a surface region (right-hand portion n FIG. 11) where only a small amount of resin 58 is present is ground excessively to cause even the metal layer 52 to be ground, thus causing the metal layer 52 to become thinner and, in the worst case, causing the substrate 50 to be exposed.
It is accordingly an object of the present invention to provide a method of producing a printed circuit board which enables, when through holes of through via conductors formed in a base is filled with resin paste and cured by heating, the surface of cured resin formed on the base to become nearly flat, thus making it possible to remove an unnecessary portion of the cured resin with ease and efficiency and therefore making it possible to obtain a high yield rate.
It is a further object of the present invention to provide a mask which is used in a method of producing a printed circuit board, of the foregoing character.
To accomplish the above objects, the present invention provides a method of producing a printed circuit board having a base provided with through via conductors having through holes closed by resin fillings. The method comprises preparing the base having the through via conductors having the through holes, filling the through holes of the through via conductors with resin paste in such a manner that, of the through holes, those which are arranged at smaller intervals are filled with a smaller amount of resin paste than those which are arranged at larger intervals, curing the resin paste, and removing an unnecessary portion of resin resulting from the curing of the resin paste and thereby forming the resin fillings.
The present invention further provides a mask for use in a method of producing a printed circuit board having a base provided with a plurality of through via conductors having through holes closed by resin fillings. The mask comprises a plurality of filling holes aligned with the through holes of the through via conductors, respectively, of the filling holes, those which are arranged at smaller intervals having a smaller cross sectional area than those which are arranged at larger intervals.